At the present time, soldering remains the preferred method for attaching each conductive member (e.g., a lead or termination) of an electronic component to a corresponding metallized region on a printed circuit board. Currently, there is a trend towards smaller electronic components which have reduced-size conductive members and a corresponding reduction in the size of the metallized regions on the printed circuit board to which such conductive members are soldered. Reducing the size of the component's conductive members and the size of the metallized regions on the circuit board reduces the area of contact between each conductive member and the circuit board, making it increasingly necessary to achieve a high-quality solder joint between them.
An important criterion in achieving a high-quality solder joint between each conductive member of the component and the corresponding metallized region on the circuit board is the solderability of the conductive member and metallized region. The solderability of the conductive members of the component and the metallized region on the circuit board is determined by the extent to which solder "wets" them (i.e., the extent to which the solder adheres to them). For this reason, sample lots of components and circuit boards are commonly tested to determine their solderability.
In the past, the solderability of sample lots of components and printed circuit boards (collectively referred to as "samples") has been evaluated by fluxing the samples and then immersing them in a bath of molten solder. After immersion, the sample is withdrawn and then visually inspected to determine the extent to which solder has wetted it. As may be appreciated, this technique for evaluating solderability is highly subjective and offers no accurate mechanism for discerning small variations in the solderability of identical samples.
Recently, testers have been developed for objectively measuring solderability in accordance with the wetting force of solder on the sample. Upon immersion of the fluxed sample into the molten solder bath, the solder wets the metallic portions of the sample (e.g., the leads of the component or the metallized regions on the printed circuit board). As the solder wets the metallic portions of the sample, the sample is subjected to a wetting force which overcomes the buoyancy force opposing immersion of the sample into the bath. By the same token, the wetting force of the solder on the sample increases the amount of force required to withdraw the sample once it has been immersed. By measuring the wetting force on the sample during immersion into, and withdrawal from, the solder bath, an accurate measure of the solderability of the sample can be obtained.
Present day solderability testers typically employ a linear variable differential transformer (LVDT) to measure the wetting force of solder on a sample. In practice, an "alligator"-type clip is used to secure the sample to the LVDT so that the sample overlies a bath of molten solder held in a heated pot. The pot is driven upwards and downwards to and from the sample by a stepper motor so that the sample can be immersed into, and withdrawn from, the solder bath. As the sample is immersed into, and withdrawn from, the solder path, the LVDT measures the wetting force on the sample, thereby providing an indication of its solderability.
A disadvantage of the above-described solderability testers is that the alligator clip used to secure the sample to the LVDT usually cannot accommodate very small samples, such as discrete surface-mounted resistors and capacitors. Further, the prior art alligator clip, by its very design, often contributes to inaccurate solderability measurement as a result of its inability to hold certain types of samples such that only a limited part of its metallic portion is immersed into a bath of solder in a vertical manner. Depending on the geometry of the sample, only a small part of its metallic portion is ultimately soldered during assembly. This is particularly true with those surface-mounted electronic components which have an arcuate toe at the lowermost end of their conductive members. Only the lower portion of the toe is ultimately soldered to a metallized region on the circuit board. Since only the lower portion of the toe, especially its bottom surface, is ultimately soldered, its solderability (rather than the solderability of the rest of the conductive member) is really of concern. With the prior art alligator clip, it is usually impossible to hold the sample so that only the toe of the conductive member may be immersed to obtain an exact solderability measurement.
Thus, there is a need for an improved clip or holder for securing a component or printed circuit board during solderability testing.